It can become easy (at least for me) to get stuck in Analysis Paralysis. If this then this, maybe, but not always, but if I .... you know what I mean. Sounds like you're actively there. Lots of us have switched several times and found something that we think works best for us. I'm not sure you can get to 100% sure - so quit trying (my advice)
I can make as many arguments for starting/staying on Avonex as I can for Rebif. And if you've read around this board, many folks would recommend neither.
But I say (just an MSer - Not a Doc - I'm a computer geek) that if you've got the diagnosis - get him on one of the disease modifying drugs ASAP. He can always change if it's not going well for him later.
For myself. I chose Rebif after taking Avonex for a long time - then having a nasti attack. I decided I wanted to try something "stronger". Most folks and docs will tell you the choice between Beta or Rebif is sort of a wash. I wish you both the best.
Hugz n Blessed Be )O(
P.S. He's not going to enjoy the initial doses of Interferon no matter which one you select.
I'm learning to breathe under water. It's quite beautiful down here.
I used to think that if I tried, I would drown.